Shares of Johnson & Johnson /zigman2/quotes/201724570/composite JNJ -1.76% gained 1.7% in premarket trading on Thursday, the day after a study published in the New England Journal of Medicine showed that its COVID-19 vaccine candidate generated neutralizing antibody titers in 100% of roughly 800 adult participants younger than 55 years old by day 57. (Some of the data from the Phase 1/2 clinical trial was published in preprint form in September.) "In addition, during 71 days of follow-up after the first dose, antibody titers further increased and stabilized, which suggests durability of the Ad26.COV2.S-elicited immune response," the researchers wrote in the NEJM. The company's experimental COVID-19 vaccine candidate is a single-dose vaccine, though the Phase 1/2 trial also includes a cohort of participants who receives two doses. It is also currently being tested in a Phase 3 clinical trial. J&J said it plans to share additional data from this trial about the immune response in adults older than 55 later this month, and it also expects to release topline data from the Phase 3 trial sometime in January. Two vaccines have been authorized by the Food and Drug Administration in the U.S., both of which are mRNA vaccines that require two doses. J&J is developing an adenovirus-based vaccine. J&J's stock is up 7.7% over the past year, while the S&P 500 /zigman2/quotes/210599714/realtime SPX -1.31% has gained 15.8%.